In the pulp and paper industry hydrocyclones are widely used for cleaning fibre suspensions from undesired particles and pollutants, most commonly heavy particles. Thus the fibre suspension is separated into a heavy fraction containing, said undesired heavy particles and a light fraction containing fibres.
In the definition of undesired heavy particles, this comprises particles having higher density compared with the accepted fibres, such as sand, grit, metal, coating flakes and high density plastics. But the undesired particles could also be organic particles originating from wood sources, for example various bark particles, shives, chops, resin particles, vessels and thick wall coarse fibres. The latter ones could have equal density as accepted fibres but is separated due to its lower specific surface.
A typical hydrocyclone plant for this purpose has hydrocyclones arranged in cascade feedback stages.
In order to keep the number of feedback stages down it is important to separate with as high selectivity as possible within each hydrocyclone, i.e. minimize the fibre portion separated and discharged through a heavy fraction outlet of each hydrocyclone, without reducing the share of undesired particles. It is also important to reduce the fibre concentration in the heavy fraction outlet in order to avoid clogging of the heavy fraction outlet at the apex and obtain secure operation conditions. A smooth inside surface of the hydrocyclone may be used to obtain a good dirt removal, as this allows the particles to migrate to the hydrocyclone wall with as moderate disturbing turbulence as possible. However, this will at the same time increase the amount of rejected fibres that settles towards the hydrocyclone wall. Thus, the Thickening factor Tf,Tf=Rm/Rv 
where Rm is Reject share by mass (ratio of fibres) and Rv is Reject share by volume (ratio of the flow) taken out at the heavy fraction outlet, is large for hydrocyclones with smooth inner surfaces. A high Tf is not wanted as it greatly increases the risk of operational problems including fibres blocking the reject outlet, high fiber losses towards the reject outlet, locally higher fibre consistencies resulting in fibre networks or flocks that traps the heavy particles that is to be separated. A high Tf will further result in higher costs due the fibre reject losses or alternatively, in an aim to resolve the problem, due to higher energy and investment costs by using cascade coupled hydrocyclones for fibre recovery purposes. Consequently an aim is to minimize the thickening factor Tf.
In order to minimize the Thickening factor of a hydrocyclone, means for creating turbulence may be provided in the separation chamber. Such examples are described in, for example, EP 615469B1. Such means for creating turbulence may be a step where the radius of the inside wall of the separation chamber suddenly increases, which causes a turbulent flow expanding flocks of fibres and releasing undesired particles from the fibre network often forming close to the wall of the separation chamber. The steps are parallel with the centre axis of the hydrocyclone.
But there is a need of balancing so that the creating of a turbulent flow expanding fibre flocks will not disturb the helical vortex separating the undesired particles so that the separation efficiency of the hydrocyclone will not be diminished by for instance a larger share of remixing of either unseparated or remixing of already separated heavy particles into the hydrocyclone accept stream of light accept fibres. Due to the sudden increase of the radius in EP615469 there is a substantial risk that the already separated heavy particles are again remixed with the light fraction.
Another known hydrocyclone having means for creating turbulence is Celleco Cleanpac 130 made and sold by GL&V Sweden AB. It has a helical path in the circumferential wall of the separation chamber, along a portion of the separation chamber, in the same direction as a helical vortex of the liquid stream when in use. The means for creating turbulence is the similar as in EP 615469 B 1, i.e. the helical path shows a sudden increase in radius of the separation chamber, one per revolution of the helical path and parallel with the centre axis.
A further known hydrocyclone is described U.S. Pat. No. 4,153,558, having axially oriented guide bars. Although these guide bars have a decreasing radius in the separation chamber, their extensive lengths in an axial direction disturb the helical vortex and the separation efficiency is decreased. Additionally, the fact that the guide bars are axially oriented and lack any components in the direction of the liquid stream further prevents the hydrocyclones capability to transport the heavy particles towards the reject outlet.